The wheel may be the most important mechanical invention of all time. However, the wheel suffers some disadvantages because it is only optimized for use on level surfaces. Climbing stairs or negotiating bumpy terrain with a wheel(s) induces large shocks or accelerations and/or may require significantly more force or torque to accomplish compared to travelling over substantially flat terrain.
Attempts have been made to address stair climbing problems associated with conventional circular wheels by replacing the conventional wheel with some other arrangement. For example, three smaller circular wheels have been mounted in a generally triangular shape. This configuration suffers the distinct disadvantage of being difficult to turn on level ground.